There have been proposed a number of amorphous semiconductor deposited films such as amorphous silicon semiconductor deposited films containing hydrogen atoms or/and halogen atoms, amorphous silicon carbide semiconductor deposited films containing hydrogen atoms or/and halogen atoms, amorphous silicon nitride semiconductor deposited films containing hydrogen atoms or/and halogen atoms, amorphous silicon germanide semiconductor deposited films containing hydrogen atoms or/and halogen atoms, amorphous germanium semiconductor deposited films containing hydrogen atoms or/and halogen atoms, etc. which are usable as element members for semiconductor devices, electrophoto-graphic photosensitive devices, image input line sensors, image pickup devices, photovoltaic devices, other various electronic elements and optical elements. Some of these amorphous semiconductor films have been practically used.
For the formation of these amorphous semiconductor films, a plasma CVD method by glow discharge in a reaction gas with the use of a high-frequency (RF) to cause formation of a deposited film on a substrate of glass, quartz, heat-resistant synthetic resin, aluminum, stainless steel, etc. has been evaluated as being the most desirable (this method will be hereinafter referred to as "RF glow discharge decomposition method") .
Various apparatus have been proposed for practicing said RF glow discharge decomposition method. However, there are unsolved problems for the RF glow discharge decomposition method that film deposition rate is low; gas utilization efficiency is low; the gas pressure upon forming a deposited film is required to be high and because of this, a raw material gas is liable to polymerize within a vapor phase when fine particles are caused; maintenance of the apparatus is not easy; and the product eventually becomes costly.
In recent years, a MW-PCVD method has been noticed on the industrial scale as a new film-forming method which can eliminate those problems of the RF glow discharge decomposition method and which can be replaced said method.
For instance, a ECR microwave plasma CVD method and an apparatus therefor are disclosed in Journal of Non-Crystal-line Solids, Vol. 77 & 78, 813-816 (1985). Further, in Japanese Unexamined Patent Publication No. 186849/1985, there is disclosed a MW-PCVD apparatus having an internal reaction space formed by a plurality of cylindrical substrates being arranged so as to circumscribe a microwave introducing means which can heighten the utilization efficiency of a raw material gas.
The known MW-PCVD apparatus has such a constitution as shown in a schematic vertical section view of FIG. 4. Referring to FIG. 4, the apparatus comprises a film-forming chamber 401 comprising a circumferential wall having an end portion thereof hermatically provided with a microwave introducing window (microwave transmissive window) 402 made of a dielectric material such as alumina ceramics, quartz, etc. to which a waveguide 403 extending from a microwave power source 411 through an isolator (not shown) is connected, a microwave power meter 410 and a stub tuner 409. The film-forming chamber 401 has a discharge space (reaction space) 406 and a plurality of rotatable cylindrical substrate holders 407 therein. On each of said substrate holders 407, there is placed a cylindrical substrate 405 on which a film is to be formed. Said plurality of rotatable cylindrical substrate holders 407 are concentrically arranged in the film-forming chamber 401 so as to circumscribe the discharge space 406. Each of the rotatable cylindrical substrate holders 407 contains an electric heater 407' for heating the substrate 405 placed thereon. Each of the rotatable cylindrical substrate holders 407 is supported by a rotary shaft 408 connected to a drive motor (not shown). Numeral reference 404 stands for an exhaust pipe for evacuating the film-forming chamber which is connected through a main vacuum valve (not shown) to a vacuum pump (not shown). The film-forming chamber 401 is provided with means for supplying a film-forming raw material gas into the film-forming chamber 401 (not shown).
The film-forming chamber 401 is so designed as to start discharge by self-exciting discharge without using a discharge trigger or the like, and it has a cavity resonator structure capable of resonating with an oscillation frequency of a microwave power source.
The formation of a deposited film by using the apparatus shown in FIG. 4 is carried out as follows.
The inside of the film-forming chamber 401 is evacuated through the exhaust pipe 404, and the heater 407' is actuated to heat each of the substrates 405 to a predetermined temperature and it is maintained at this temperature. Concurrently, the substrates 405 are rotated by the drive motors respectively at a desired constant speed. Then, in the case of forming an amorphous silicon deposited film, raw material gases such as silane gas and hydrogen gas are fed through the gas supply means into the film-forming chamber 401. At the same time, a microwave having a frequency of 500 MHz or more, preferably, 2.45 GHz is caused from the microwave power source 411. The microwave is supplied through the waveguide 403 and the microwave introducing window 402 into the film-forming chamber 401. Thus, the raw material gases thus introduced into the film-forming chamber 401 are excited and decomposed with the action the microwave energy to cause neutral radicals, ions, electrons, etc. Then, these are reacted with each other to form a deposited film on each of the substrates 405.
For the conventional MW-PCVD apparatus, there are problems that films are unavoidably deposited on the surface of the microwave introducing window 402 which is situated in the film-forming chamber as film formation proceeds, and along with this the quantity of a microwave energy applied into the film-forming chamber is gradually changed to cause negative effects such as insufficient decomposition of a raw material gas, reduction in the film deposition rate, reduction in the utilization efficiency of the raw material gas, etc. In addition, said films deposited on the surface of the microwave introducing window are sometimes removed in fine particles which get into a film to be deposited on the substrate and as a result, the resulting film becomes such that is accompanied with defects. In order to avoid occurrence of these problems, it is required to periodically clean the microwave introducing window or to periodically replace it by a new one. In this connection, the product unavoidably becomes costly.
Particularly, in the case of preparing an electrophotographic photosensitive device, since its light receiving layer is usually such that has a thickness of, for example, 20 to 40 .mu.m, it takes a long period of time for the formation thereof even at a high deposition rate in any case. Because of this, a relatively large amount of films is deposited on the surface of the microwave introduced during the film forming process and the quantity of a microwave energy applied is changed accordingly. In the case when the film-forming process is repeated. As a result, the resulting layer assumes to have such electric characteristics as varied in the thickness direction. Further in the above case, the microwave introducing window is markedly heated because, in addition to receiving a radiant heat caused by plasma generated in the discharge space, absorption of microwave by said films deposited on the surface of the microwave introducing window, when said films are accordingly heated. This situation is less problematic when said films are thin. But it is problematic when they are thick since the quantity of microwave absorbed by them becomes significantly large and said films are heated to an elevated temperature to case phase change such as crystallization within them. In this case, the quantity of a microwave energy to be applied into the film-forming chamber is largely varied to make the state of discharge to be caused in the discharge space unstable and as a result, as above described, the resulting layer becomes such that is accompanied with defects. In addition, the microwave introducing window is sometimes damaged because of being overheated.
In view of the above, there is an increased demand to make an effective improvement to eliminate the foregoing problems on the conventional MW-PCVD apparatus and to provide a desirable MW-PCVD apparatus which enables one to stably and repeatedly produce a functional deposited film while utilizing various advantages of the MW-PCVD film-forming method.